Novolak (or novolac) resins are commonly used as the polymeric component of lithographic compositions, such as photoresist compositions used in the manufacture of semiconductors. Novolak polymers prepared by conventional synthesis methods are mixtures of polymers formed by the acid catalyzed condensation reaction of a molar excess of a phenol, having at least two of its ortho and para positions relative to the hydroxyl group unsubstituted, with formaldehyde. The reaction proceeds in two steps. The rate limiting (slow) step involves the addition of formaldehyde to the unsubstituted ortho and para positions on the phenol ring. No ring substitution occurs at the meta position. In the much faster step, the methylol groups, resulting from formaldehyde addition to the phenol ring during the rate limiting step, are joined with the excess phenol at its unsubstituted ortho and para positions by methylene bridges therebetween forming the novolak polymer. The sequence of reactions may be represented by the following equation (1); the novolak shown being only one of the many possible configurations actually formed in the complex mixture of structures and stereoisomers. ##STR1##
Phenolic Resins, A. Knop and L. A. Pilato, Chapter 3, Springer-Verlag, NY (1985) provides a more detailed discussion of novolak resins.
The novolak polymers, like other condensation polymers, have a broad molecular weight distribution. This distribution becomes broader as the desired molecular weight of novolak polymers increases. The molecular weight of novolak polymers prepared by conventional synthesis techniques is controlled by the ratio of formaldehyde to phenol. The conventional broad molecular weight distribution is a direct consequence of the single step growth nature of the above synthesis in which each methylol substituted phenol reacts with a stoichiometric amount of the excess phenol. In an open system, the ability to reproducibly prepare novolak polymers having the same molecular weight becomes increasingly difficult as the molecular weight of the polymer is increased. The molecular weight distribution is difficult to reproduce because the high volatility of formaldehyde decreases the effective ratio of formaldehyde to phenol present for the rate limiting step of the reaction. In addition, for all practical purposes, the stereochemistry of such a single step growth polymer is random, and therefore, it is very difficult to utilize the conventional synthesis process to reproducibly manufacture high molecular weight novolak polymers of a consistent molecular weight and composition.
A very important criterion for selecting a polymer for use in lithographic applications is the dissolution rate of the polymer in the developer solution. The developer, typically an organic solvent or an aqueous base solution, is used to selectively remove portions of a polymeric coating after certain portions of the coating have been exposed to actinic radiation. In a positive acting photoresist composition the developer is used to selectively remove those portions of the photoresist film which have been exposed to the actinic radiation. In a negative acting photoresist composition the developer selectively removes those portions of the photoresist film which have not been exposed. The lithographic performance of a photoresist is a function of the photoresist dissolution rate expressed in terms of sensitivity and contrast. Sensitivity refers to the dose of exposing radiation needed to achieve a specified dissolution rate difference between the exposed and unexposed polymer. Lithographic potential is measured as the logarithm of the fraction of the unexposed dissolution rate of the photoresist divided by the dissolution rate of the exposed photoresist film. Alternatively, sensitivity may be expressed in terms of the lithographic potential of the photoresist at a constant exposure dosage. Contrast refers to the slope of a plot of the lithographic potential (vertical axis) as a function of the exposure dose (horizontal axis); the higher the sensitivity and contrast the better the lithographic performance of a polymer.
I evaluated the lithographic performance of a number of novolak polymers prepared by conventional synthesis techniques and hypothesized that the lithographic performance could be improved if the extent of branching of the novolak were to be increased. Further, I also noted that the absorbance by novolaks of deep ultraviolet radiation in the wavelength range of from about 235 to 300 nanometers appeared to decrease as the concentration of p-cresol used as one of the phenols in the novolak synthesis increased, however, conventional p-cresol-containing novolaks formed from greater than about 40 weight percent p-cresol were not sufficiently soluble in aqueous base solutions for use in photoresists.